Telephone line protection devices have been required for many years by telephone operating companies, and the like, in order to provide protection to central office switching equipment from electrical strikes and power line voltages coupled to the telephone lines. At least one telephone line protector is required for each telephone subscriber line. In view that many central office switching systems service thousands, and often hundreds of thousands of subscribers, many such protection modules are required. Each module is constructed of a standardized size and with five pins of a uniform configuration. Two pins are associated with the tip and ring telephone lines, two other pins are associated with the customer or equipment tip and ring lines, and a fifth pin is connected to ground.
The basic function of a telephone line surge protection circuit is to sense an excessive voltage, such as 300 volts, on either the telephone line tip or ring conductor, and very quickly connect such conductor to ground to prevent the customer equipment from experiencing sustained voltages higher than such threshold. Specialized solid state devices are generally utilized for providing a high speed short circuit to ground. A host of other subsidiary features can be carried out by the protection device, including fail-safe grounding in response to a thermal overload, open circuiting in response to overcurrent conditions between the telephone line and customer lines, etc.
Because of the high demand for protection devices, a myriad of such devices are currently available. Illustrative of the many different protection device designs and modules are those set forth in U.S. Pat. No. 4,796,150 by Dickey et al.; U.S. Pat. No. 5,031,067 by Kidd et al; U.S. Pat. No. 5,172,296 by Kaczmarek; and U.S. Pat. No. 5,357,568 by Pelegris. In all of these protection module designs, there are many complicated parts which are necessary to carry out the line protection functions.
In many of the conventional telephone line protection modules, encapsulated semiconductor devices are utilized as overvoltage sensors and switches. The reason for this is that such devices are generally available from the manufacturer only in packaged or encapsulated form. Again, the additional processing of the devices to package the same results in a more expensive component, often when packaged devices are not necessary. For example, when a semiconductor device is packaged so as to have pins, then often a corresponding socket is required. It can be appreciated that the use of packaged components often leads to a more expensive product.
In many of the telephone line protection devices, including those identified in U.S. Pat. Nos. 5,101,317, 5,341,270, and 5,359,657, primary overvoltage protection is provided by fast-acting solid state devices, many of which are obtainable from Teccor Electronics, LP of Irving, Tex. These solid state devices are often referred to as “SIDACtor® devices”, and are described more fully in U.S. Pat. No. 5,479,031 which is assigned to Teccor Electronics, LP. Secondary or fail-safe protection is often provided by other mechanical apparatus that is part of the telephone line protection module. U.S. Pat. Nos. 5,175,662, 5,357,568 and 3,359,657 disclose telephone line protection modules employing failsafe mechanisms. The fail-safe apparatus is responsive to the thermal energy generated as a result of a sustained overvoltage condition imposed on the telephone line conductor(s). This apparatus generally involves a material having a low-temperature melting point, such as a tin-lead solder which, when subjected to a temperature that causes melting thereof, enables a spring to move and short circuit the tip and ring line conductors to the ground terminal of the module. The melting point of the solder compounds generally used may be as low as 150° F. Low-temperature melting solders are desirable because they trigger the fail-safe mechanism before extremely large sustained currents destroy other parts or components connected to the telephone line. However, low-temperature solders can in some instances melt due to elevated temperatures in indoor or outdoor cabinets and enclosures, in absence of any overvoltage on the telephone line conductors. This inadvertent activation is obviously undesirable.
The basic primary overvoltage protection and the fail-safe mechanism can be fabricated in a host of different variations, many of which are extremely complicated and thus expensive. Aside from the cost considerations of a mechanically complex device, the reliability is a function of numerous mechanical and electrical components. It can be appreciated that as the number of components increase and the component complexity increases, the assembly time and cost also increase.
From the foregoing, it can be seen that it would be advantageous to provide a new type of protection module that has few components, is easily assembled and is much less costly than the prior art modules. It would be advantageous to have an improved telephone line protection module that employs a semiconductor device assembly that is easily replaceable. Another need exists for a protection module design utilizing conductor tower strips of different shapes to achieve various functions, such as test points, eliminating series line current protection, and the connection therebetween of various circuit components. Another need exists for a protection module that incorporates a simplified failsafe mechanism.